Binding patterns of fluorescein isothiocyanate (FITC)- and gold-conjugated lectins to extracellularly melanized sheathed and exsheathed microfilariae of subperiodic Brugia malayi, isolated from and in situ in the abdominal hemocoel of Anopheles quadrimaculatus 72-hr postinfection, were examined. Five FITC-conjugated lectins [Helix pomatia agglutinin (HPA), Arachis hypogaea (peanut agglutinin-PNA), Triticum vulgaris (wheat germ agglutinin-WGA), Lens culinaris (lentil-LCH), and Concanavalin A (Con A)] with specificities for different carbohydrate moieties were tested for binding to isolated melanized microfilariae and observed with transmitted light and fluorescence microscopy. All five FITC-lectins bound strongly to the acellular material accompanying the melanin deposits on the surface of isolated melanized microfilariae. Significant inhibition of FITC-lectin binding occurred when lectins were preincubated with their complementary carbohydrates before testing. H. pomatia agglutinin binding was totally inhibited by N-acetyl-D-glucosamine and N-acetyl-D-galactosamine. Other lectins were partially inhibited, such as PNA by galactose and lactose; WGA by N-acetylneuraminic acid; LCH by N-acetyl-D-glucosamine, mannose, glucose, and methyl alpha-D-mannopyranoside; and Con A by mannose and methyl alpha-D-mannopyranoside. Three gold-conjugated lectins (HPA, PNA, and Con A), examined by using transmission electron microscopy, bound to the outer surface of the acellular material associated with the melanin deposits on isolated melanized microfilarial sheaths and melanized microfilariae and to the remnants of lysed hemocytes found in the proximity of the melanized deposits. Con A in the presence of gold-labeled horseradish peroxidase, examined by using transmission electron microscopy, showed random binding within the melanized capsule formed around the microfilarial sheath in situ. These results indicate that the acellular material accompanying melanin deposits on melanized microfilarial sheaths and sheathed and exsheathed microfilariae contain several glycoconjugates with exposed carbohydrate moieties and are possibly glycoproteins. These glycoproteins could be the by-products of the activation of the prophenoloxidase by the microfilariae.